Gas hydrate refers to a solid material which is formed when gas such as methane (CH4) is combined with water molecules (H2O) at a low temperature and high pressure of 0° C. and 26 atmospheres or 10° C. and 76 atmospheres. Gas hydrate is easily found in an area adjacent to an oil or natural gas reservoir and a coal bed in a frozen soil region or a low-temperature and high-pressure deep-sea sedimentary layer, or particularly a continental slope.
In order to utilize such gas hydrate as a resource, an advanced mining technology must be applied. When the pressure is lowered, gas hydrate is dissociated while releasing methane. Thus, it is difficult to mine gas hydrate in a solid state like coal. As a method for extracting only methane by dissociating hydrate, various methods are used, which includes a depressurization method, a thermal injection method, an inhibitor injection method, a replacement method and the like.
According to the depressurization method, a borehole is formed in a free gas layer adjacent to gas hydrate so as to reduce the pressure of the gas layer. As the pressure of the free gas layer is reduced, the hydrate of the gas hydrate layer is dissociated to generate gas.
According to the thermal injection method, steam or hot water is injected to increase the temperature of a gas hydrate reservoir. Then, hydrate is dissociated to generate gas. The thermal injection method may be considered when there is no free gas layer adjacent to gas hydrate.
According to the inhibitor injection method based on a technology which is used to prevent hydration in a cold region, an additive such as methanol or glycol is injected to change a dissociation condition. When only the inhibitor injection method is used, a significant effect may not be obtained. However, when a hydraulic fracturing method and the thermal injection method are used at the same time, the effect of the inhibitor injection method is expected to be improved. However, the inhibitor injection method has disadvantages in that environmental pollution is likely to occur and the economic efficiency thereof is low due to a high cost required for a solvent used therein.
According to the replacement method which is a method for altering the molecular structure of gas hydrate, captured methane is extracted by replacing methane within gas hydrate with another material. When the replacement method is used, methane may be produced without melting a gas hydrate layer.
In addition, the method for extracting only methane by dissociating hydrate includes a geothermal stimulation method which generates hot water using ground heat and injects the generated hot water, and a controlled oxidation method which dissociates hydrate through a catalytic oxidation reaction in a stratum.
The region abundant in gas hydrate may be roughly divided into two regions. In general, a large amount of gas hydrate is found in the permanently-frozen soil and the continental slopes in the deep ocean. Depending on where gas hydrate is buried, the difficulty level of recovery may differ. For example, when gas hydrate exists in the hard rocks, ground deformation or ground subsidence hardly occurs while the gas hydrate is recovered. However, when gas hydrate exists in unconsolidated strata in the sea, ground deformation or ground subsidence may occur while the gas hydrate is recovered. Thus, it is important to previously analyze a ground deformation characteristic on gas hydrate recovery through an experiment, and to estimate the extent to which the strata is deformed, based on the result obtained through the experiment. In the current technical field related to gas hydrate, there has been proposed only a method and apparatus for recovering gas hydrate or an apparatus for artificially generating gas hydrate as disclosed in Korean Patent Laid-open Publication No. 10-2009-0122812. However, an experiment apparatus capable of estimating ground deformation through observation during gas hydrate recovery has not yet been disclosed.